CVE-2021-21408 is a high-severity (CVSS 8.8) Improper Input Validation vulnerability in smarty/smarty. A fix is available for smarty/smarty — see the affected versions and patch details below.
Access to restricted PHP code by dynamic static class access in smarty
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2021-21408.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-21408 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
smarty/smarty🐘smarty/smartyReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
Impact
Template authors could run restricted static php methods.
Patches
Please upgrade to 3.1.40 or higher.
References
See the documentation on Smarty security features on the static_classes access filter.
For more information
If you have any questions or comments about this advisory please open an issue in the Smarty repo
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | smarty/smarty | all versions | 3.1.43composer require smarty/smarty:^3.1.43 |
| 🐘Packagist | smarty/smarty | ≥ 4.0.0&&< 4.0.3 | 4.0.3composer require smarty/smarty:^4.0.3 |
Affected Products
debian linuxdebianfedorafedoraprojectsmartysmartyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for smarty/smarty, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update smarty/smarty to 3.1.43 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21408 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2021-21408 in your dependencies?
Find it across Packagist, including transitive dependencies.